The valve seat is a portion serving as a valve seat for an intake valve or an exhaust valve necessary for keeping a combustion chamber airtight in contact with a valve face. Major functions of a valve seat include (1) an airtight function, i.e. prevention of a compressed gas or a combustion gas from leaking to a manifold, (2) a thermal conduction function, i.e. releasing of the heat from a valve to a cylinder head, and (3) a wear resistance function, i.e. resistant against to a collision in a valve seating and an wear in an high temperature and a high load situation. In addition, characteristics required on a valve seat include (1) low opposite aggressivility on a valve face, (2) reasonable price, and (3) easy machinability. Therefore an iron-based sintered alloy material is applied to a valve seat of an internal combustion engine to satisfy the above-mentioned functions and characteristics.
An iron-based sintered alloy material is obtained by the compression moulding in which a metal powder or the like is put into a metal mould followed by heating of the powder mould at a temperature equal to or lower than the melting point, and will be subjected to a heat treatment or otherwise if required. The iron-based sintered alloy material is made advantageous by containing suitable amounts of carbon, copper, nickel and the like in addition to iron as a main component in (1) mechanical properties, wear resistance, heat resistance and the like are improved by elements mixed in order to improve wear resistance of a sintered alloy, (2) machinability of a product is improved, (3) cost reduction by improved productivity is achieved, and the else.
However, specifications required on the materials for constituting automobile parts have been made severe year by year as well as to other various machines, i.e. further improvement on mechanical characteristics, workability such as machinability and stabile productivity and reduction of manufacturing cost are required. As for a valve seat, it is not exceptional and valve seats for internal combustion engines having better characteristics than mechanical characteristics of conventional valve seats for internal combustion engines have been required.
As response to such requirements, Patent Document 1 discloses a valve seat excellent in wear resistance with made poor opposite aggressivility to a valve face in which 10 to 20% by area in area ratio of a first hard particle which is a cobalt-based intermetallic compound particle having a particle diameter of 10 to 150 μm and of a hardness equal to or higher than 500HV0.1 and less than 800HV0.1 is made to included, and 15 to 35% by area in area ratio of a second hard particle which is a cobalt-based intermetallic compound particle having a particle diameter of 10 to 150 μm and a hardness equal to or higher than 800HV0.1 and less than 1100HV0.1, and make the total area ratio occupied by the both dispersed in an iron matrix to be 25 to 55% by area.    [Patent Document 1] Japanese Patent Laid-Open No. 2005-248234